Realize it is not uncommon to fill off road equipment tires with water so this is as far fetched as is sounds. There is no rule specifically against this so it comes down to a judgement call to decide if it is properly contained. IE not likely to spill on the field. Same class as marbles, sand, BBs, and so on. Do keep in mind that this is coming from a team that never puts a robot on the field without a container full of lead & sulfuric acid.
As for as slime goes. We ran it in our Frisbee shooter flywheel to give it mass & balance. Got a controls award for the accuracy of the shooter at one regional. Good thing nobody ask Q&A is it was legal that year.
